1. Field of the Invention
The present invention relates to drive devices for a light emitting display panel in which a light emitting element constituting a pixel is actively driven by a TFT (thin film transistor) and in which a reverse bias voltage can be applied to the light emitting element, and particularly to drive methods and drive devices for an active type light emitting display panel in which deterioration in light-emitting efficiency of the light emitting element accompanied by applying of the reverse bias voltage and the like can be compensated.
2. Description of the Related Art
A display using a display panel which is constructed by arranging light emitting elements in a matrix pattern has been developed widely. As the light emitting element employed in such a display panel, an organic EL (electro-luminescent) element in which an organic material is employed in a light emitting layer has attracted attention. This is because of backgrounds one of which is that by employing, in a light emitting layer of an EL element, an organic compound which enables an excellent light emitting characteristic to be expected, a high efficiency and a long life have been achieved which make an EL element satisfactorily practicable.
As display panels in which such organic EL elements are employed, a simple matrix type display panel in which EL elements are simply arranged in a matrix pattern and an active matrix type display panel in which an active element consisting of a TFT is added to each of EL elements arranged in a matrix pattern have been proposed. The latter active matrix type display panel can realize low power consumption, compared to the former simple matrix type display panel, and has characteristics such as less cross talk between pixels and the like, thereby being specifically suitable for a high definition display constituting a large screen.
FIG. 1 shows one example of a most basic circuit configuration corresponding to one pixel 10 in a conventional active matrix type display panel, which is called a conductance control technique. In FIG. 1, a gate of a controlling TFT (Tr1) comprised of N-channels is connected to a scan line extending from a scan driver 1, and its source is connected to a data line extending from a data driver 2. A drain of the controlling TFT connected to a gate of a driving TFT (Tr2) comprised of P-channels and to one terminal of a capacitor C1 provided for holding electrical charges.
A source of the driving TFT (Tr2) is connected to the other terminal of the capacitor C1 and to an anode side power supply (VHanod) supplying a drive current to an EL element E1 provided as the light emitting element. A drain of the driving TFT (Tr2) is connected to an anode of the EL element E1, and a cathode of this EL element is connected to a cathode side power supply (VLcath) via a switch SW1. This example shown in FIG. 1 is constructed also in such a way that a reverse bias voltage source (VHbb) can be applied to the cathode of the EL element via the switch SW1 as will be explained later.
In the structure shown in FIG. 1, when an ON controlling voltage (Select) is supplied to the gate of the controlling TFT (Tr1) via the scan line, the controlling TFT (Tr1) allows current which matches the voltage (Vdata) supplied from the data line to the source to flow from the source to the drain. Therefore, during the period when the gate of the controlling TFT (Tr1) is at an ON voltage, the capacitor C1 is charged, and the capacitor's voltage is supplied to the gate of the driving TFT (Tr2) as a gate voltage. Thus, the driving TFT (Tr2) allows current based on its gate-to-source voltage (Vgs) to flow through the EL element E1 to drive the EL element so that the EL element emits light.
It is well known that the organic EL element electrically has a light emitting element having a diode characteristic and an electrostatic capacity (parasitic capacitance) connected in parallel thereto, and it has been known that the organic EL element emits light whose intensity is approximately proportional to the forward current of the diode characteristic. It has been also known empirically that by applying a voltage one after another in a reverse direction (reverse bias voltage) which does not participate in light emission to the EL element, the life of the EL element can be prolonged.
The structure shown in FIG. 1 is constructed in such a way that a forward or reverse bias voltage can be applied to the EL element E1, utilizing the switch SW1. That is, an electrical potential relationship among the anode side power supply (VHanod), the cathode side power supply (VLcath), and the reverse bias voltage source (VHbb) is set to VHbb>VHanod>VLcath. Therefore, in the state of the switch SW1 shown in FIG. 1, a forward voltage of the value of (VHanod−VLcath) is supplied to a series circuit of the driving TFT (Tr2) and the EL element E1. When the switch SW1 shown in FIG. 1 is switched to the opposite direction, a reverse bias voltage of the value of (VHbb−VHanod) is supplied to the series circuit of the driving TFT (Tr2) and the EL element E1.
FIG. 2 also, similarly, shows a conventional example constructed in such a manner that the reverse bias voltage can be applied to the EL element, and this example also shows the case where the conductance control technique is applied. In FIG. 2, portions corresponding to the respective portions explained based on FIG. 1 are designated by like reference numerals, and therefore individual explanation thereof will be omitted. The example shown in this FIG. 2 is constructed in such a manner that first and second change-over switches SW1, SW2 are provided so that by switching the switches SW1, SW2, a connection relationship of the anode side power supply (VHanod) and the cathode side power supply (VLcath) is switched.
That is, in the case where the switches SW1, SW2 are in the state shown in the drawing, the forward voltage of the value of (VHanod−VLcath) is supplied to the series circuit of the driving TFT (Tr2) and the EL element E1. Thus, the forward current can be supplied to the EL element E1, and the EL element E1 can be brought to a lighting state by an ON operation of the driving TFT (Tr2). When the switches SW1, SW2 are switched to the directions opposite to that of the drawing, similarly, the reverse bias voltage of the value of (VHanod−VLcath) is supplied to the series circuit of the driving TFT (Tr2) and the EL element E1. A structure of the case where the VLcath is used as a reference potential (ground voltage) is disclosed in Patent Reference 1.
Japanese Patent Application Laid-Open No. 2002-169510 (paragraph Nos. 0001 and 0012, FIG. 2, and the like).
Meanwhile, since the organic EL element is a current light emitting type element, in general, a constant current drive is performed for the driving TFT. The EL element has a predetermined parasitic capacitance as described above, and further the EL element is brought to a light emitting state when a predetermined light emission threshold voltage or greater is given thereto. Thus, even when a drive voltage is applied to the EL element in a forward direction, since electrical charges are charged into the parasitic capacitance, a predetermined time is necessary to reach the light emission threshold voltage. Furthermore, since the constant current drive is performed as described above, its impedance is substantially high, and therefore rising to the light emission threshold voltage of the EL element necessitates a longer time.
In addition, in the case where the above-described means for applying the reverse bias voltage to the EL element is adopted, since electrical charges are accumulated in a reverse bias state in the parasitic capacitance of the EL element, a time period from a time when the forward voltage is applied to a time when the EL element is brought to the light emitting state is further necessary. Thus, a lighting time rate of an EL element decreases, thereby resulting in a substantially deteriorated light-emitting efficiency. Problems that respective EL elements are affected by variations in times that are until EL elements are brought to the light emitting state and the like and therefore linearity of gradation control is deteriorated and the like occur.